I know we like pictures hanging straight, doors that open and close without sticking, along with all the other things we like plumb, level or square, but I think many miss the purpose of the three point support of the surface plate.
If the plate is supported as it's intended then no amount of being out of level will affect it's indended purpose of checking for straight, flat, etc. I wouldn't build a stand or support system out of square or want it leaning but out of level by a 1/16", there's no harm done. I'm not saying there's harm done (psychological is the exception
) by wanting to level the plate but it isn't necessary. Instruments for measuring square, angles, heights etc. don't come equiped with a bubble for a reason.